The invention concerns melt-spun polyethylene fibres having a high strength.
A conventional method for manufacturing fibres from synthetic polymers is spinning with spinnerettes, where a polymer brought in flowing state is pressed through holes and the fibres being formed are stretched, whereby the fibres become thinner and an orientation of molecular chains takes place in the longitudinal direction of the fibres. Depending on how the polymer used in the manufacturing of the fibres is brought into a flowing state, fibres are referred to, for example, as solution-spun, gel-spun and melt-spun fibres.
Mixtures of a polymer and a solvent are used as raw material for fibres both in solution spinning and gel spinning, which in view of the industrial production of the fibres is a drawback resulting in increased costs. Firstly, in the fibre production it is necessary to use extra chemicals (solvents, swelling agents), the adding and recovery of which from the fibres cause additional costs. Secondly, the presence of extra chemicals in the fibre manufacturing process decreases the production capacity. Although at present fibres can be produced by gel-spinning polyethylene fibres having a strength comparable to that of carbon fibres or even greater, the high manufacturing costs of this kind of fibres prevent wider usage of the fibres in industrial applications.
Compared to the former methods the melt spinning is simpler, because the polymer is simply melted in an extruder and pressed through holes. The extra costs and drawbacks related to the use of additional chemicals can thereby be avoided. By melt spinning it is also possible to produce polyethylene fibres at sufficently high production rate.
Recently a lot of research work has been carried out for developing processes for the production of high strength polyethylene fibres to replace higher priced carbon fibres and LCP-fibres. Very many factors have influence on the quality of the fibres obtained and on the strength properties. Such factors are, among others, the properties of the polyethylene used as raw material, the spinning conditions such as the temperature, the dimensions and contours of the spinning holes and especially the conditions used in the stretching. Among the properties of the fibre raw material, the molecular weight has appeared to be especially important.
Concerning the prior art, reference is also made, among others to GB-patent publications 1498628 and 1568964. In the method according to the first mentioned patent polyethylene having a weight average molecular weight of between 50000-200000 is extruded into fibres, which are cooled to a temperature of 100.degree.-120.degree. C. at a rate of 1.degree.-15.degree. C. per minute and the fibres are cooled rapidly thereafter. After that the fibres are stretched at a temperature, which is at least 40.degree. C. below the melting point by using a draw ratio of at least 18. This process enables however a very slow spinning due to the slow cooling step. The spinning rates disclosed in the patent are thereby only 4-5 m/min. Also the fibre strength obtained by the method is not very high. In the latter GB-patent there is used polyethylene having a weight average molecular weight of greater than 150000 and the ratio of weight average molecular weight to the number average molecular weight is greater than 5.
Concerning the prior art, reference is also made to U.S. Pat. No. 4,228,118, in which polyethylene having a number average molecular weight of at least 20000 and a weight average molecular weight below 125000 is extruded at a temperature of 220.degree.-335.degree. C. The molten fibres are cooled and the fibres obtained are stretched at a rate, which is at least 30 m/min. After that the fibres are hot-stretched at a draw ratio of at least 20:1 at a temperature of 115.degree.-132.degree. C. It is aimed by this method to obtain fibres having a strength of at least 12 g/den (10,6 cN/dtex).
The information concerning the factors affecting fibre strength and interactions between them is very confusing and incomplete. Only some of the affecting factors are generally known. For example it is known that by increasing the molecular weight of the fibre raw material, fibres having a higher strength are basically obtained. Likewise it is known that stronger fibres can be achieved by stretching. Fibre strength has also been tried to be improved by using different stretching conditions.
When the aim has been fibres having a strength higher than that of the fibres obtained according to the prior art, the known solutions have not given results. The molecular weight of the fibres cannot be increased freely, because thereby the melt spinning of the fibres becomes impossible. The stretching of the fibres has also limits, which cannot be exceeded without impairing some or all properties of the fibres.